dpdkstat: fix retrieval of statistics

dpdkstat: fix retrieval of statistics

This change forces dpdkstat plugin to check if memory needs to be resized on
every DPDK_CMD_GET_STATS command, rather than resizing it only once at
initialization. Additionally it fixes incorrect metric retrieval logic, which
allows plugin to dispatch metrics only if number of retrieved extended statistics
is equal to allocated stats array size for port. Based on DPDK API documentation
rte_eth_xstats_get (and rte_eth_xstats_get_names) function can return positive
value lower than array size, which isn't considered a failure.

If primary DPDK process is restarted with a greater number of ports bound to
DPDK driver while collectd dpdkstat plugin is running, then memory isn't resized.
Due to insufficient memory allocation dpdkstat plugin is unable to gather metrics
from new ports.

Change-Id: I25c8995105a322474653bf7065c2228047f886b1
Signed-off-by: Przemyslaw Szczerbik <przemyslawx.szczerbik@intel.com>